The Long Term Evolution (LTE) system of 3GPP standardization organization supports both Frequency Division Duplex (FDD) mode and Time Division Duplex (TDD) mode. For the LTE TDD, each radio frame that occupies a length of 10 ms is equally divided into two half-frames each having a length of 5 ms. The LTE TDD frame structure is shown in FIG. 1. Furthermore, each half-frame contains three special domains as well as eight timeslots each having a length of 0.5 ms. These three special domains include a DL pilot timeslot (DwPTS), a guard period (GP) and a UL pilot timeslot (UpPTS), which, in total, have a length of 1 ms. Additionally, each subframe in the LTE TDD frame structure shown in FIG. 1 consists of two consecutive timeslots; that is, the kth subframe contains a timeslot 2k and a timeslot 2k+1.
The LTE TDD supports seven UL-DL configuration modes, as shown in FIG. 1. In FIG. 1, D represents a downlink (DL) subframe, U represents an uplink (UL) subframe, and S represents a special subframe containing the three special domains mentioned above.
According to the regulations as defined by the present LTE protocol, the TDD UL-DL configuration of a cell is semi-static and thus will not be frequently changed by an eNB. At least, the TDD UL-DL configuration in the cell remains unchanged during one time of data transmission.
TABLE 1ConfigurationSwitchingSubframe No.Mode No.Period01234567890 5 msDSUUUDSUUU1 5 msDSUUDDSUUD2 5 msDSUDDDSUDD310 msDSUUUDDDDD410 msDSUUDDDDDD510 msDSUDDDDDDD610 msDSUUUDSUUD
In an existing LTE/LTE-A system, the UE periodically or aperiodicly feeds back CSI according to the configuration of the eNB so that the eNB can acquire the DL channel condition of the UE. The aperiodic CSI feedback is triggered by a CSI request bit(s) contained in an UL Downlink Control Information (DCI) format. The LTE-A system under 3GPP Standard Protocol Rel. 11 defines ten Transmission Modes (TM). For TM1-9, no CSI Process is involved or the number of involved CSI Processes can be considered as 1. For TM10, the eNB can allocate a plurality of CSI processes to the UE by means of high-layer signaling, and each of these CSI processes can, at the most, be configured with two CSI subframe sets. In one or more CSI processes triggered by one aperiodic CSI feedback, when one CSI process is configured with two CSI subframe sets, one aperiodic CSI feedback triggers CSI feedback specific to one of said two subframe sets CCSI,i (i=0 or 1). In an embodiment, the UE determines a CSI reference resource in one of the DL subframes of CCSI,i, and further measures the CSI; wherein CCSI,i is determined by a DL subframe where a CSI request that triggers above-mentioned aperiodic CSI feedback is located; that is, CCSI,i is a CSI subframe set which contains the subframe where the UL DCI containing the CSI request is located. Thus it can be seen that, according to the 3GPP Standard Protocol Rel. 11, the subframe set, corresponding to the aperiodic CSI that is fed back, is indicated implicitly by a DL subframe where the UL DCI containing the aperiodic CSI request is located.
In order to meet the increasing demands of mobile users, the flexible TDD reconfiguration technology in LTE-A system receives more and more attentions gradually. The flexible TDD reconfiguration technology allows the ratio of current UL subframes to DL subframes adapting to the ratio of current UL traffic load to DL traffic load, in a better way, so as to facilitate the improvement of UL-DL peak rate, and hence increase the throughput capacity of the system.
For a flexible TDD reconfiguration system, the TDD/UL/DL configuration of a cell is dynamically varying depending on the current UL-DL traffic load of the cell, which means that parts of subframes may dynamically change their transmission direction and hence become flexible subframes. In addition, the radio environment of a flexible TDD reconfiguration cell may be influenced by neighbor cells with common frequency or adjacent frequency which usually work at TDD mode, resulting in that parts of DL subframes in the flexible TDD reconfiguration cell are heavily influenced by a neighbor cell which is primarily attributing to the inter-cell interference (hereinafter referred to as primary neighbor cell), e.g., when the subframes on these subframe locations in said primary neighbor are also DL subframes; while the other subframes, usually flexible subframes, are weakly influenced by the primary neighbor cell, e.g., when the subframes on these subframe locations in said primary neighbor cell are UL subframes. In this way, the DL subframes of each radio frame in a flexible TDD reconfiguration cell, including the flexible subframes, may be divided into two subframe sets, CH and CL, wherein each of the two sets may contain only one element.
For sake of description, the examples that take the DL subframe set heavily influenced by the primary neighbor cell as CH and take the DL subframe set weakly influenced by the primary neighbor cell as CL are illustrated. For instance, it is necessary to feedback CSI for the above-mentioned two sets, respectively, so as to take full advantages of the flexible TDD reconfiguration technology.
However, in order to reduce the complexity of DL scheduling sequence, the flexible TDD reconfiguration system may sometimes not transmit UL DCI format through subframes pertaining to CL, because if so, such subframes, usually flexible subframes, may require readjusting the UL scheduling sequence when the flexible subframes mentioned above are switched into UL subframes; moreover, even if it is allowed to transmit the UL DCI through subframes pertaining to CL, the UL subframes corresponding to these subframes, sometimes, may be switched into DL subframes (e.g., when subframe #4 and subframe #9 are pertaining to CL while subframe #8 and subframe #3 are flexibly switched into DL subframes), then it may be unable to feedback aperiodic CSI specific to CL. This means that the aperiodic CSI feedback specific to CL may not be triggered if it continues to use the method defined by 3GPP Standard Protocol Rel. 11, i.e., implicitly indicating the subframe set corresponding to the aperiodic CSI that is fed back by a DL subframe where the UL DCI containing the aperiodic CSI request is located.
Thus it can be seen from the analysis above that, for a flexible TDD reconfiguration cell, in order to realize aperiodic CSI feedback specific to CH and CL, the method defined by 3GPP Standard Protocol Rel. 11 will be confronted with problems that the aperiodic CSI feedback specific to the flexible DL subframes cannot be triggered, which has not been well solved yet, by now.